Add configurable DEM sources and centerline file imports - #2

Summary

This adds configurable elevation (DEM) and centerline sources to the RiverREM server workflow. I wanted a way to use local DEM files (either uploaded or via a local folder) and additional centerline GIS files (geopkg and shp). The 2 additional DEM options are not enabled by default; the thinking being if this is deployed online a server admin may not want to allow users to upload large files or configure a folder.

Note: this branch includes commit 3bed64c (my pending color-relief fix). This PR is stacked on my earlier color-relief PR. If that PR merges first, GitHub should automatically remove the already-merged commit from this PR’s changes.

DEM sources

• Add streamed local GeoTIFF uploads.
• Add an optional read-only server DEM library.
• Retain support for Mapterhorn and remote COG URLs.
• Hide upload/library options unless enabled by the server.
• Automatically remove expired uploaded DEMs.
• Limit custom DEM processing size to reduce memory failures.
• Check custom DEM, viewport, and centerline overlap before processing.

Centerlines

• Add GeoJSON, GeoPackage, and zipped Shapefile imports.
• Assume GeoJSON uses EPSG:4326.
• Detect and reproject CRS-aware GeoPackages and Shapefiles.
• Prompt for a CRS when metadata is missing.
• Support layer selection for multi-layer files.
• Show filename, CRS, feature count, coordinate count, length, and import warnings.
• Validate Shapefile components, geometry, coordinates, and complexity.
• Ignore macOS ZIP metadata such as __MACOSX and AppleDouble files.

Documentation

• Add setup and usage instructions to the main README.
• Add detailed DEM-source and centerline-import documentation.
• Document the new environment variables and Docker configuration.

Testing

• Frontend production build passes.
• All 10 backend tests pass.
• Tested GeoJSON, GeoPackage, and macOS-created Shapefile ZIP imports.
• Tested EPSG:6344 reprojection against a custom DEM.
• Tested successful and non-overlapping custom DEM/centerline combinations.

This branch is currently stacked on the previously submitted color-relief display fix.